1. Field of the Invention
The present invention relates to a color image signal processing method and apparatus for obtaining color signals for recording a color image from input color image signals.
2. Related Art
FIG. 1 is a block diagram showing the main part of a conventional ink-jet printer for recording a color image using video signals. R(red), G(green) and B(blue) analog input signals are converted into digital signals in A/D converter 11 and are then inputted to gamma converters 12R, 12G and 12B to be subjected to gradation conversion and complementary conversion, thereby obtaining gradation converted complementary color signals C, M and Y.
The complementary color signals C, M and Y are subjected to masking processing at a masking ROM 14 for the removal of ink colorless components (uneven color components) to obtain signals C', M' and Y'. The signals C', M' and Y' representative of ink optical densities (OD values) are converted into analog drive voltages by an OD-V converter 16 to obtain appropriate OD values. The analog drive voltages drive heads 18C, 18M and 18Y. The heads eject ink droplets 20C, 20M and 20Y to record a desired color image on a recording medium (not shown).
The masking ROM is constructed of a masking conversion table. However, it is difficult in practice to make the capacity of the table small since the color reproduction precision depends on the table capacity.
In the technology handling color image signals of subtractive mixture, a black signal produced from yellow (Ye), magenta (M) and cyan (Cy) signals of equal quantity does not follow the theory faithfully. Also, subtractive primaries are expensive. Because of the above and other reasons, it is common to form a color image using four color image data including an additional black signal (Bk).
FIG. 2 shows an example of a block for generating a black signal according to the prior art. The block including a black generation table 201 made of ROMs is applied to an ink-jet printer, which is shown in FIG. 3. The black generation table 201 shown in FIG. 2 is used to generate a black signal with a luminance signal (Y) and two color difference signals (R-Y) and (B-Y) inputted thereto. Upon input of signals Y, R-Y and B-Y, the table 201 outputs a signal Bk which has been previously calculated and stored therein. The calculation of Bk is as follows: ##EQU1## where R, G and B are additive primaries of red, green and blue, respectively.
The values Bk calculated as above are stored for each combination of Y, R-Y and B-Y.
In the conventional examples shown in FIGS. 2 and 3, it is necessary to input three signals Y, R-Y and B-Y to the black generation table 201. Thus, the capacity of the table made of, for example, ROMs becomes large, and hence the whole circuit becomes bulky. The process of Bk=Min(Cy, M, Ye) is fundamental to the under color removal so that the above problem of large capacity arises not only for the combination of Y, R-Y and B-Y but also for the other color specification systems.
To solve the above disadvantages, the present applicant has proposed in U.S. application Ser. Nos. 585,602 and 029,386 the technology of reducing the memory capacity by changing the number of masking input bits in accordance with color signal components. Masking process using luminance Y, hue H and chroma C or using R-Y, B-Y and luminance Y has also been proposed in U.S. application Ser. Nos. 838,785 and 838,784, respectively.
The present invention provides an improved technology of the above conventional proposals.